1. Field of the Invention
The present invention relates to a magnetic material detecting device adapted to detect a concentration of magnetic material.
2. Description of the Related Art
In a developing device provided in electrophotographic or electrostatic recording image forming apparatus, a two-component developer composed predominantly of toner particles and carrier particles is widely used. In particular, a color image forming apparatus for forming full-color or multi-color images often uses a two-component developing device as the developing device in terms of hue of image or the like.
As is well known, a toner concentration of the two-component developer, i.e. the proportion of weight of toner particles to the total weight of carrier particles and toner particles is a highly important factor in stabilizing image quality.
Toner particles in the developer are consumed on developing process. This causes a change in a toner concentration thereof. Therefore, a developer concentration controller is used to correctly detect the toner concentration of the developer timely, and implement toner replenishment depending on the change, thereby controlling the toner concentration so as to always keep it constant, resulting in retaining image quality.
For detection of a toner concentration, a magnetic material detecting device is widely used, which has no problem about toner stain and can stably detect the concentration.
Apparent magnetic permeability of a developer changes depending on the mixing ratio of magnetic carriers (carrier particles having magnetism) added to non-magnetic toner. The magnetic material detecting device, then, detects the apparent magnetic permeability of the developer, and then detects a toner concentration in the developing device on the basis of the result.
Detection of large apparent magnetic permeability of the developer, for example, can exhibit a state of the increased proportion of the magnetic carrier in the developer occupied within a certain volume, i.e. a state of a low toner concentration. In this case, an image forming apparatus starts toner replenishment.
On the contrary, detection of small apparent magnetic permeability can exhibit a state of the reduced proportion of the magnetic carrier in the developer occupied within the certain volume, i.e. a state of a high toner concentration. In this case, the image forming apparatus stops the toner replenishment.
A device employing a differential transformer is known as the above magnetic material detecting device. For example, a downsized magnetic material detecting device as shown in FIG. 6 has been proposed (for example, Japanese Laid-Open Patent Publication (Kokai) No. 11-190933).
FIG. 6 is a schematic diagram of a magnetic material detecting device according to the prior art.
As shown in FIG. 6, a differential transformer 100 has differentially connected drive coils 104 and 105, and an output coil 106.
An alternating current signal generator 101 supplies an input signal (an alternating current signal) Ein with predetermined electrical characteristics to the drive coils 104 and 105 to drive the drive coils 104 and 105. The input signal Ein from the alternating current signal generator 101 is also supplied to an output coil 106 via a resistance R1 to stabilize an output signal E outputted by the output coil 106.
An amplifier 102 amplifies the output signal E outputted from the output coil 106. An output signal Eout outputted from the amplifier 102 has a phase difference depending on the concentration of magnetic material relative to the input signal Ein.
A phase difference detector 103 detects a phase difference between the input signal Ein supplied to the drive coils 104 and 105 and the output signal Eout outputted from the output coil 106 via the amplifier 102. The adjustment control unit 107 detects the concentration of the magnetic material based on the input signal Ein generated by the alternating current signal generator 101 and the phase difference detected by the phase difference detector 103.
However, the conventional magnetic material detecting device shown in FIG. 6 has a problem that a phase difference signal outputted depending on the concentration of magnetic material easily varies depending on a fluid state of the magnetic material even when the concentration of the magnetic material is constant.
In other words, even when the concentration of the magnetic material is constant, flow of the developer with unevenness (condensation and rarefaction) in density distribution of the magnetic material produces change in a phase difference value detected by the magnetic material detecting device depending on a timing of detecting the phase difference.
This leads to false recognition that the concentration of the magnetic material has changed, regardless of the constant concentration of the magnetic material.